As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users are information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes, thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems, e.g., computer, personal computer workstation, portable computer, computer server, print server, network router, network hub, network switch, storage area network disk array, RAID disk system and telecommunications switch.
In the information handling systems, printed circuit board subassemblies have signal buses comprising a plurality of conductive traces formed on the printed circuit board insulated substrate. Integrated circuit bus controllers drive data signals over these plurality of conductive traces with very little margin over a minimum specified receiver input level, e.g., specified receiver mask requirements. Printed circuit boards have a wide variation in conductive trace characteristics depending upon board design, materials and processes used, manufacturing tolerances, etc. This is especially prevalent in variations of the conductive trace cross-sectional geometries. Since the DC resistance of the conductive traces are proportional to the cross-sectional geometry, trace resistance may vary even from the same manufacture using the board design. Where the printed circuit board subassembly is operating under very little signal voltage margin, an increase of as little as one ohm of DC trace resistance may make the difference between an acceptable received signal level or a receiver signal mask violation.
Printed circuit board signal handling specifications have focused on trace impedance rather than variations in trace resistance. For example, Intel's Hub Interface (HI) specifies an RCOMP resistor for impedance compensation. RCOMP has a tolerance of one percent or better and is used to set a reference resistance against which the on-die bus driver termination compensates. The value of the RCOMP resistor may change depending upon the characteristic impedance of the data bus. For example, HI 2.0 specifies a 62.5 ohm resistor for a 50 ohm characteristic bus impedance, and a 75 ohm resistor for a 60 ohm characteristic bus impedance. However, variations of the trace resistance among printed circuit boards, even of the same design, are not taken into account. True impedance of the traces on the printed circuit board are dismissed as a minor variance. A well used solution to higher signal loss (higher resistance) traces has been to increase signal drive strength for all bus signals. This results, however, in increased power draw and heat dissipation. It may also increase cross-talk interference between the signal traces.
Another solution in avoiding higher then desired DC trace resistance-caused problems is to increase the cross sectional area of the traces, e.g., widen and/or thicken (increase height of) the traces. This may increase the cost of printed circuit board layers and ultimately the cost of the printed circuit board assembly. Another possible but costly solution is to require the printed circuit board manufacturers to measure DC trace resistance for each printed circuit board, similar to the inspection process used for high end impedance-controlled printed circuit boards that require measurement of trace impedance. Requiring this type of inspection may reduce the number of printed circuit boards that pass and thus will ultimately increase the cost of these printed circuit boards.
None of the aforementioned solutions would successfully address another problem where data buses pass through integrated circuits, e.g., exchange switches, having a wide variation of resistances in the signal paths.